Q. We are expanding some of our buildings and need to replace the existing fire pumps. What are the technical requirements for such pumps?

A. Pumps currently used for fire protection in buildings are primarily centrifugal and provide water to a sprinkler and/or standpipe system. NFPA (National Fire Protection Association) issues Standard for the Installation of Centrifugal Fire Pumps, which deals with the selection and installation of pumps supplying liquid for private fire protection. The scope of this document includes liquid supplies; suction, discharge and auxiliary equipment; power supplies, including power supply arrangements; electric drive and control; diesel engine drive and control; steam turbine drive and control; and acceptance tests and operation. The standard applies to centrifugal single-stage and multistage pumps of the horizontal or vertical shaft design and positive displacements pumps of the horizontal or vertical shaft design. A Technical Committee on Fire Pumps, consisting of a broad range of interested parties involved with fire protection, reviews and updates this document (referred to as pamphlet NFPA 20) on a three-year cycle.

NFPA published its first standard for automatic sprinklers in 1896, and through the workings of the Technical Committee on Fire Pumps, each edition of NFPA 20 has incorporated appropriate provisions to cover new developments and has omitted obsolete provisions. NFPA 20 2007 has been approved by ANSI and is used not only as a national standard but is also accepted internationally.

Fire pumps have rated flow rates ranging in discreet flow increments from 6-m3/h (25-gpm) through 1135-m3/h (5000-gpm), with net pressures starting from 275-kPa (40-psi) and depending on the manufacturer, are capable of up to 4410-kPa (640-psi). For a centrifugal pump to be qualified as a fire pump, it must meet stringent mechanical and hydraulic requirements that are witnessed and certified by Underwriters Laboratories (UL) or FM Global, two independent third-party testing agencies. These testing agencies have established (with input from industry experts) specific engineering requirements that a pump must meet before it can be listed or approved as satisfactory for fire service. The Authority Having Jurisdiction (AHJ) is the organization, office, or individual responsible for approving equipment such as a fire pump when used for the protection of life and property.

Double suction axial split case pumps, close-coupled vertical in-line pumps, and horizontal end suction pumps can be used for the majority of fire pump applications. If an installation has a static suction lift, then a vertical turbine type pump must be utilized. The primary drivers for fire pumps are electric motors or diesel engines, and almost all are started automatically.

Q. Our plant operates a number of centrifugal pumps with ball bearings, and bearing failure is the most common cause of unscheduled pump shutdown. What should be the average life of pump bearings and what can be done to improve it?

A. Centrifugal pump bearings, selected from bearing manufacturer's catalogs, provide a minimum life of two years. That means that 90 percent of the bearings will last that long. The average life will be five times that, or ten years.

However, other factors affect bearing life and can shorten it appreciably. Contamination and poor lubrication are the most common culprits, especially in process service. Water and process liquid can enter the bearing housing and disrupt the lubrication process. More effective shaft seals at the bearing housing can help reduce this problem. Frequent monitoring of the lubricant purity will also help.

Excessive bearing load can also reduce life. Operating the pump at rates of flow below 50 percent of the best efficiency flow imposes increased radial forces on the bearing. Coupling misalignment can also be a contributing factor.

This subject is discussed in great detail in the latest edition of ANSI/HI 1.3, Rotodynamic (Centrifugal) Pumps for Design and Application.

Q. How can the bathtub vortex at the bottom of a pump suction tank be eliminated as the liquid level is drawn down?

A. Since a dry-pit pump is usually located some distance downstream from its piping inlet in the tank, the effect of these flow disturbances on the pump is not as severe as with wet-pit pumps. For example, local flow separation, swirl or velocity non-uniformities, although creating greater head losses at the inlet, may be dissipated in the approach piping to the pump. The main problem is usually entrainment of air (or other tank gases) due to free-surface vortices, as this air may collect in the piping (causing air binding) or cause degra­dation of pump performance.

Preventing the formation of free-surface vortices at tank inlets to pumps allows the tank to be drawn to lower levels than would otherwise be possible.

Additional information on this subject can be found in HI Standard ANSI/HI 9.8, Pump Intake Design.

Pumps & Systems, February 2009